The micro structure of donor doped BaTiO_3 ceramics was observed via light microscopy utilizing an oblique incident beam (OIB) applied to the polished surface of the sample. The source of light (halogen lamp) for OEB method was added under sample holder and optical fibers were used as light leaders on surface of sample. The angle of incidence was optimalized during observation till the maximum of contrast was obtained (region of angles between 40-60 deg). It's known that the square of refraction index is equal to the dielectric permitivity of material in the VIS region of spectrum. The dielectric permitivity of BT is anisotropic and the index of refraction is not the same in different crystallographic directions. This causes birefrigence of light beam. The value of BT relative dielectric permitivity is strongly influenced by grain size. For BT ceramics with grain size > 1 mu m (the smallest grain size in prepared BT ceramics), BT relative dielectric permitivity value inreases with decreasing grain size. In the real material, the reflected surface is not perfectly smooth and the fraction of the reflected light is given by addition of specular and diffuse reflection. The diffuse reflactance depends on roughness of reflected surface and scattering coefficient in the case of opaque material. The microstructure image of polished BT ceramics observed by SEM had low contrast and boundaries of large grains were hardly distinguished. In microstructure image of BT ceramics observed by OIB method, large grains reflect the beam less than small grains and the intensity of reflected light decreases with grain size. Separate grains are well distinguished and it's evident that oblique arrangement of the light source to the sample surface improves the final microstructure image.
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